A photovoltaic array outputs DC voltage. This voltage depends on such factors as solar irradiance, temperature, and the electrical load presented to the array.
To be more commercially useful, one provides this DC voltage to a utility class solar photovoltaic inverter. The inverter converts the DC photovoltaic array output into an AC voltage, which is then stepped up with a transformer and provided to a typical AC utility grid. In addition, the inverter maintains a maximum power point tracker (MPPT) for causing the photovoltaic array to operate at its maximum power point.
Occasionally, the inverter may fail and require maintenance. Or an alarm may trip, and the inverter protections will have to be reset. In either case, the inverter is disabled, and therefore no longer interacts with the photovoltaic array. Once this occurs, the inverter no longer controls the load. As a result, the photovoltaic array's output DC voltage increases.
Eventually, the inverter must be re-enabled so that the photovoltaic array can be put back into service. A difficulty can arise, however, if the output voltage of the array is higher than the maximum operating voltage of the inverter's switching elements. This is particularly likely at cold ambient temperatures. In such cases, re-enabling the inverter may expose the switching elements to excessively high voltage, and thus cause them to fail.
Another problem arises when, at the time the inverter is to be re-enabled, the ambient air temperature is at or near the minimum operating temperature of the switching elements. This arises because as the temperature falls, the breakdown voltage associated with the switching elements also falls. For example, at −40° C., the breakdown voltage of a switching element may be 5%-15% below its breakdown voltage at 25° C.
One solution is to reduce the array's DC voltage output. However, the output voltage depends on factors that are difficult to control, such as solar irradiance and outdoor temperature. It is difficult to temporarily shield the array from the sun, or to change the outdoor temperature so that the inverter can be re-enabled.
A known way to safely re-enable the inverter is to wait until dark, or until the temperature rises sufficiently. However, this is often inconvenient both because of the delay, the resulting lost revenue from electricity production, and the need to employ personnel after normal working hours. Moreover, in extreme latitudes, where days are long, it may be weeks or months until the sky is dark enough to reduce the voltage sufficiently to re-enable the inverter. And at those times when the sky is dark for extended periods, it may be weeks or months before the temperature rises enough to safely re-enable the inverter.